Motor vehicles have pneumatically operated devices, for example, compressed air brake systems, which have to be supplied with compressed air. The compressed air is usually provided by a compressor that sucks in air from the atmosphere and outputs a compressed air flow into the pressure line. Since the air moisture contained in the air condenses and can cause corrosion, which considerably shortens the service life of components, the compressed air is dried after leaving the compressor, before it is fed into the line network. For this purpose, a dryer unit is arranged in the pressure line. It is known to arrange feed control elements at the inlet of the pressure line in order to switch the compressed air delivery by the compressor on and off, and to control the pressure in the pressure line and therefore in the consumer network. The feed control element can be a control input of the compressor, which is actuated by way of compressed air.
DE 196 38 837B4 discloses a pressure medium preparation device that provides a compressor for supplying pressure medium and a 3/2-way valve as a feed control element assigned to the compressor. The continuous compressed air flow that is produced can be fed via the feed control valve either to those elements of the pressure preparation device that are connected downstream or to atmosphere. The feed control valve can be actuated by compressed air by means of its control input. In the known device, an air dryer and a non-return valve are connected downstream of the feed control valve. Here, a multiple-circuit protective valve is connected to the non-return valve as well as individual pressure medium circuits of the pressure medium system. A pressure regulating device is connected to the control input of the feed control valve, and can connect a store line that branches off from the pressure line downstream of the non-return valve to the control input of the feed control valve, in order to suppress the compressed air supply to the pressure line. In the known arrangement, the pressure regulating device consists of a pressure-actuatable governor valve and an electrically actuatable control valve. A first control input of the governor valve is connected to the store line and operates counter to a permanent restoring force of a valve spring. The output of the governor valve is connected to the feed control valve. The electrically actuatable control valve is likewise connected by way of its input to the store line and is connected by way of its output to a second control input of the governor valve, the second control input acting counter to the first control input of the governor valve. The electrically actuatable control valve or its control input is connected via an electrical line to an electronic control unit. The control unit generates a control signal for controlling the valve as a function of a pressure measurement in the store line and therefore the pressure line.
The electrically actuatable valve acts as pilot controller of the governor valve and causes a switchover of the governor valve for acting on the compressed air feed into the pressure line. As soon as the electrically actuated valve connects the second control input of the governor valve to the store line, the control forces of the two pressure-controlled inputs of the governor valve cancel one another out and the restoring force of its valve spring switches the governor valve into the open position. In this switching position, the store line is connected to the feed control valve at the compressor, with the result that the compressed air feed into the pressure line is disconnected and the compressed air flow from the compressor is fed to atmosphere.
Although, in the known device, the pressure control takes place during normal operation via the electrically actuatable valve, the known device can maintain an emergency mode in the event of a power failure and therefore a failure of the control valve, the feed control taking place solely in a pressure-dependent manner. In the event of a power failure, the electrically actuatable valve is switched into the non-open position, with the result that the second control input of the governor valve is likewise not actuated. The governor valve then connects the store line to the feed control valve if the pressure in the pressure line and therefore the store line overcomes the counteracting spring force via the first control input of the governor valve. The provided governor switchover pressure that is maintained by the governor via corresponding switching of the compressed air feed is defined by the setting of the restoring force of the valve spring of the governor valve.
Although, in the event of a failure of the electrical control energy, the known compressed air preparation device can maintain pressure-dependent control of the working pressure via the governor valve in an emergency mode, unrestricted journey continuation of the motor vehicle is not possible by way of the known compressed air preparation device, since the quality of the compressed air preparation decreases very rapidly after a failure of the electrical control energy.